The present invention relates to a planar light source device, a display device and an optical reflection sheet.
As an image display device for a personal computer or other various monitors, use of liquid crystal display devices has been widespread. In general, a liquid crystal display device has a liquid crystal display panel and a backlight unit disposed on a back thereof. The liquid crystal display panel displays an image by controlling light transmitted therethrough. Among some types of liquid crystal display devices, there is one having a sidelight type backlight unit. The sidelight type backlight unit typically includes a cold cathode fluorescent tube as a light source disposed on a side of a display surface of the device.
FIG. 1 is a side elevational view showing a liquid crystal display 10 having a sidelight type backlight unit. In this drawing, reference numeral 11 denotes a backlight unit. Numeral 12 denotes a liquid crystal display panel to which a drive circuit is fitted. Numeral 13 denotes a diffusion sheet for diffusing light to even brightness on a surface of the liquid crystal display panel. Numeral 14 denotes a prism sheet for collecting light to improve luminance on the front of the display panel. Numeral 15 denotes a light guide plate for guiding and diffusing light from the display's light source. Numeral 16 denotes a cold cathode fluorescent tube which serves as a linear light source. Numeral 17 denotes a lamp reflector for reflecting light from the cold cathode fluorescent tube, and numeral 18 denotes a reflection sheet reflecting light, which is disposed on a back of the light guide plate.
On back lower surface 15c of the light guide plate is formed a dot pattern made of white ink or consisting of concave or convex structural portions. Light from cold cathode tube 16 passes through a light incident surface 15a into the light guide plate 15 and propagates therein. Since the refractive index of the light guide plate is greater than that of air, light would not be emitted from the upper surface 15b of the light guide plate without the patterned back (lower) surface 15c. Such a pattern on back surface 15c changes the angle of light propagation in the light guide plate and causes the light to be emitted from light emitting surface 15b. 
Light incident on back surface 15c is reflected (in a scattered manner) by the provided pattern. Thus, the angle of the light is changed, and constant light is emitted from upper surface 15b. Light having the angle thereof changed is partially made incident again onto surface 15c, transmitted through the surface 15c, reflected by reflection sheet 18, and made incident again into light guide plate 15. Light made incident onto concave or convex structural portions is partially reflected thereon, the other part being transmitted therethrough and also reflected by reflection sheet 18. Thus, since the angle of this light is also changed, it too is emitted from upper surface 15b. 
The dot pattern or the pattern of concave/convex structural portions is formed so that the density thereof can be varied, depending on the locations thereof. Thus, the luminance of light emitted from upper surface 15b is substantially even.
In a conventional sidelight type backlight unit, a white reflection sheet has been used. While a white reflection sheet has relatively high reflectivity, it also has many diffusion reflection components. Specular reflectance of a typical white reflection sheet is several percent. Meanwhile, a reflection sheet of metal (e.g., having a silver reflectance layer) has much higher reflectivity than such a white reflection sheet. Particularly, such a metal reflection sheet has more specular reflection components than a white reflection sheet. In a liquid crystal display, proper luminance is obviously a significant technical aspect, particularly in a liquid crystal display for use in a notebook type PC or the like. In order to achieve luminance improvement of liquid crystal displays, particularly, improvement of the front luminance thereof, a reflection sheet having many specular reflection components, and is comprised of metal, has been attempted. However, the inventors of the present invention have discovered that a problem of unevenness in luminance occurs when a silver-layered reflection sheet is used for the sidelight type backlight unit. For example, if the reflection sheet is pushed by finger from the back thereof, light emitted from the backlight unit will have uneven brightness levels. In such backlight units, the back of the frame accommodating the reflection sheet, and the light guide plate typically has large openings. These openings are made for the purpose of reducing weight of the backlight unit and thus the overall display. The back of the frame has a plurality of large openings and a plurality of beams defining the openings. Hence, when the backlight unit is mounted onto the liquid crystal display, portions of the backlight unit, which are exposed from the openings, are pressed by other parts of the liquid crystal display, or the reflection sheet is pressed by the beams. Moreover, even if the back of the frame does not have the openings, the unevenness in luminance is sometimes recognized because the reflection sheet may be warped.
In a backlight unit in which a pattern of concave/convex structural portions are formed on the guide plate and a silver reflection sheet is combined with such structural portions on the back of the light guide plate, unevenness in brightness from the emitting surface of the light guide plate resulted. In contrast, in the case of a light guide plate having a white ink dot pattern, unevenness in brightness was not visually recognized. This is believed to be because the concave/convex structural portions have a much smaller diffusion capability than the white ink dot pattern. Moreover, the problem of uneven luminance owing to the aforementioned press by a finger was more serious, especially when using a downward-facing prism sheet (as opposed to an upward-facing prism sheet). This is believed to occur because, while the upward-facing prism sheet has a function of reflecting light incident thereon to return the same to the light guide plate, the downward-facing prism sheet is used to emit light once incident thereon directly to the display panel. As a result, it was discovered that the occurrence of uneven luminance depends on the distance between the reflection sheet and the light guide plate. A transparent polyethylene terephthalate sheet was interposed between the silver reflection layer and the light guide plate, and luminance was measured while varying a thickness of the transparent sheet. It was learned that it was possible to reduce the unevenness in luminance by thickening the transparent sheet. However, it was also discovered that two problems were caused by interposing such a transparent sheet in such a manner. One was a lowering of luminance due to absorption of light by the transparent sheet. For example, in the case of interposing a sheet having a thickness of 75 micrometers, a lowering of luminance by about 6% occurred. Another is a problem of a “bright line”. The light guide plate and the reflection sheet are sandwiched by a reflector of the cold cathode tube. When a thick transparent sheet is interposed in this region, light is made incident from a side of the transparent sheet, resulting in the incident light becoming a “bright line” and appearing on the surface of the backlight unit.
Japanese Patent Laid-Open No. Hei 9(1997)-90107 discloses a technique in which glass beads are coated on the surface of the reflection sheet in the sidelight type backlight unit. Further, Japanese Patent Laid-Open No. Hei 11(1999)-64613 discloses a reflection body having an uneven layer consisting of a particle layer and a metal reflection layer formed thereon in a direct type backlight unit. However, neither of these embodiments appear to consider unevenness in luminance which occurs when using a metal reflection sheet. In the above-described documents, the glass beads and the uneven layer are formed for the purpose of diffusion and reflection of light. Accordingly, front luminance is reduced.